Interest in the hypothesized association between tuberculosis and cancer has been intensified by studies demonstrating that tubercle bacilli (i.e.BCG) potentiate host immune responses against cancer challenge. Recent experiments in man suggest that BCG acts not only to evoke non-specific cell-mediated immunity but also to increase host resistance against specific tumors such as melanoma, sarcoma and breast cancer. In view of the above, a type of natural experiment in cancer prevention is underway. A large cohort of tuberculosis patients is being followed prospectively through time to death or suuvival as of December 31, 1974. The outcome variables of interest are cancer mortality and morbidity. Observed cancer risks will be compared with those of the general population similar in age, sex and race. Additional analyses will focus upon cancer risk according to clinial severity of tuberculosis, with adjustment for smoking habit. Consideration will also be given to cancer risks among close family members as a further test of the antitumor immunogenicity of exposure to tuberculosis. The prospective design of this study is relatively free of biases inherent in earlier efforts. This will also make possible the study of the association between tuberculosis and all types of cancer. A significant relationship between the two could suggest the possibility of BCG or tuberculin vaccination as a cancer preventive.